Proteins of frog sartorius muscles were labeled by incubating muscles with either [3H]- or [14C]phenylalanine. A major, labeled protein band, apparent molecular weight approximately 96,000, was distinguishable in SDS-polyacrylamide electrophoretic gels of labeled plasma membranes. This band could be differentiated from protein bands of sarcoplasmic reticulum and mitochondrial fractions. Brief electrical stimulation of contraction of isolated, labeled muscles caused a definite, selective loss of radioactivity in the region of the plasma membrane protein band described above. We propose to study this new observation in greater detail. An attempt will be made to isolate and characterize the membrane protein that is altered by contraction. Preliminary studies have shown that an increase in concentration of Ca++ in muscles causes contracture and also leads to a diminution in amount of labeled plasma membrane protein in a manner similar to the effect of electrical stimulation. It is postulated that muscle contraction may be accompanied by activation of a Ca++ dependent protease, optimally active at neutral pH, that is known to be present in muscle and that this enzyme may attack specific membrane proteins under physiological conditions. It is known that exercise in vivo and muscle contraction in vitro are accompanied by a prolonged increase in permeability of muscle cells to sugar. An attempt will be made to see whether or not there is a correlation between changes in membrane proteins and an increase in permeability. The time course of these events and the effects of inhibitors of proteolysis will be examined. Changes in permeability to sugar are of fundamental importance for the proper nutrition and functioning of muscle. The proposed studies are part of a long-term investigation of the manner in which sugar transport is regulated in skeletal muscle cells.